Ball screw device

ABSTRACT

A ball screw device comprises a nut having a thread groove in the inner peripheral surface thereof, a screw shaft having a thread groove in the outer peripheral surface thereof and a plurality of balls interposed between the respective thread grooves. In the screw shaft, the thread groove is of at least substantially one turn. In the screw shaft is provided a ball circulation groove for coupling the downstream and upstream sides of the thread groove so that the balls are returned to the upstream side from the downstream side so as to be thereby circulated.

TECHNICAL FIELD

This invention relates to a ball screw device of ball circulation type.

BACKGROUND TECHNIQUE

A ball screw device comprises a screw shaft having a thread groove inthe outer peripheral surface thereof and a nut having a thread groove inthe inner peripheral surface thereof and externally mounted on the screwshaft. A plurality of balls are interposed between the respective threadgrooves of the screw shaft and the nut. In such a ball screw device, theballs circulate between the respective thread grooves according to therotational movement of the nut or the screw shaft, which causes arelative telescopic motion between the nut and the screw shaft. In thiscase, it is necessary, irrespective of the telescopic motion, to providea structure to prevent the balls from slipping out of between therespective thread grooves. One of such structures is a ball circulationmechanism. The ball circulation mechanism couples both ends of a ballpassageway shaped by the respective thread grooves of a screw shaft anda nut to form a closed loop, and thereby circulate the balls in theclosed loop. The conventional ball circulation mechanism employs aso-called deflector. The deflector is described below.

The deflector, in general, serves to couple the upstream side of thethread groove, which balls are rolled from, and the downstream sidethereof, which the balls are rolled to, so that the balls on thedownstream side of the thread grooves are returned to the upstream sidethereof over a ridge. The deflector is, being fitted into a radialthrough hole formed in the nut, fixed to the through hole by anadhesive. The deflector has, in the inner diameter surface thereof, aball circulation groove of meandering shape for returning the balls fromthe downstream side, over the ridge, to the upstream side insubstantially one turn of the thread grooves.

Such a ball screw device requires an externally mounted part, that isthe deflector, which generates additional costs for the deflector,processing the through hole for mounting the deflector in the nut andmounting the deflector to the nut. Besides, when the deflector ismounted to the through hole of the nut rather imprecisely, the ballcirculation groove and thread groove cannot be precisely positioned,which results in a degraded circulation performance of the balls. Thus,mounting the deflector requires precision, which is costly.

Therefore, a main object of the present invention is to provide a ballscrew device capable of circulating balls without using a deflector.

Another main object of the present invention is to provide a ball screwdevice capable of circulating balls without using a deflector, andtherefore without additional costs.

DISCLOSURE OF THE INVENTION

The ball screw device according to the present invention comprises a nuthaving a thread groove in the inner peripheral surface thereof, a screwshaft having a thread groove in the outer peripheral surface thereof anda plurality of balls interposed between the respective thread grooves.In the screw shaft is provided at least substantially one turn of thethread groove and a ball circulation groove for coupling the downstreamand upstream sides of the thread groove so that the balls are returnedfrom the downstream side to the upstream side to be thereby circulated.

In the case of the ball screw device, the ball circulation groove,instead of a deflector, is provided at a ridge of the screw shaft. Thus,such an externally mounted part as the deflector or the like can bedispensed with, which advantageously results in eliminating workingsteps such as forming a through hole for fitting the deflector to thenut and mounting the deflector thereto.

In a preferred embodiment of the present invention, in the screw shaftare provided at least two independent thread grooves axially adjacent toone another and each is of substantially one turn, and the ballcirculation grooves as many as the plural thread grooves of the screwshaft. The ball circulation grooves separately couple the respectivedownstream and upstream sides of the corresponding thread grooves of thescrew shaft. In this embodiment, the total number of the ballsincreases, and so does the load capacity accordingly.

In a preferred embodiment of the present invention, the ball circulationgrooves sink the balls, which have been rolled to the downstream side ofthe thread grooves of the screw shaft, to the inner diameter side,thereby leading the balls to travel over the ridge of the nut and reachthe upstream side of the thread grooves. In this embodiment, the ballcirculation grooves are of meandering shape so that the balls can movesmoothly between the thread grooves and the ball circulation grooves.

In a preferred embodiment of the present invention, the ball circulationgrooves respectively provided in each of the thread grooves of the screwshaft are disposed in a substantially identical phase and axiallyadjacent to one another. In this embodiment, wherein the plural ballcirculation grooves are disposed in a substantially identical phase andaxially adjacent to one another, plural turns of the thread grooves areaxially closely spaced, which advantageously reduces an axial areaoccupied by the thread grooves.

In a preferred embodiment of the present invention, a retainer ring forrotatably retaining each of the balls is mounted on the outer peripheryof the screw shaft so as to relatively rotate and in an axially fixedposition. This embodiment, wherein the retainer ring for retaining theballs is used, eliminates interference between the balls in the threadgrooves, thereby allowing easy relative rotation of the nut and thescrew shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a ball screw device accordingto a best mode for executing the present invention.

FIG. 2 is a longitudinal sectional view of the nut shown in FIG. 1having been moved in one axial direction.

FIG. 3 is a perspective view of a disassembled ball screw device.

FIG. 4 is a plan view of a part of a ball screw device shown in section.

FIG. 5 is a sectional view taken along (5)-(5) line of FIG. 4.

FIG. 6 is a side view schematically showing ball circulationpassageways.

FIG. 7 is a front view of the ball circulation passageways of FIG. 6.

BEST MODE FOR EXECUTING THE INVENTION

Referring to FIGS. 1 through 7, a ball screw device according to a bestmode for executing the present invention is hereinafter described indetail.

A ball screw device 1 comprises a nut 2, a screw shaft 3, a plurality ofballs 4 and a retainer ring 5. The balls 4 are circulated between theopposing surfaces of the nut 2 and the screw shaft 3. In the nut 2 isformed a thread groove 21 continuous from one shaft end to another shaftend. In the axially intermediate region of the screw shaft 3 is formed aplurality of—two in this embodiment—thread grooves 31 a and 31 b, whichare each of substantially one turn and independent from one another. Thethread groove 21 of the nut 2 and the thread grooves 31 a and 31 b ofthe screw shaft 3 are set at an identical lead angle. The thread grooves21, 31 a and 31 b are arranged to be of Gothic arc shape in section, butmay be semicircular in section.

The nut 2 and the screw shaft 3 are in a maximally extended state, inwhich they are separated away from one another with a maximum distancetherebetween, so that a region in which they overlap by an axiallypredetermined length is ensured. The thread grooves 31 a and 31 b of thescrew shaft 3 are arranged to be disposed in the overlapping region. Thethread grooves 31 a and 31 b are each arranged to be independent closedloops, and the balls 4, which are disposed in two turns of the threadgrooves 31 a and 31 b of closed loop, independently roll so as to bethereby circulated.

More specifically, ball circulation grooves 33 and 34 for forming thethread grooves 31 a and 31 b separately into the closed loops areprovided at a ridge 32 present between the thread grooves 31 a and 31 badjacent to one another in an axial direction of the screw shaft 3.These two ball circulation grooves 33 and 34 separately couple therespective upstream and downstream sides of the thread grooves 31 a and31 b, and each has a meandering shape for sinking the balls 4 on thedownstream side of the thread grooves 31 a and 31 b to the innerdiameter side thereby leading the balls 4 to travel over a ridge 22 ofthe nut 2 and return to the upstream side.

The retainer ring 5 serves to space the plural balls 4 atcircumferentially equal intervals to prevent the balls from interferingwith one another and is formed from a thin cylindrical member. At a fewpoints in the circumference of the retainer ring 5 are provided ballpockets 51 elliptically shaped along an axial direction, in each ofwhich two of the balls 4 are housed.

The nut 2 is integrally connected to a gear ring 7. The gear ring 7 hasa rotational power source, such as a motor, not shown, or the likeengaged thereto via a reduction gear and is supported so as to freelyrotate with respect to a support shaft, not shown, inserted through acentral hole of the screw shaft 3 via a rolling bearing, not shown. Thescrew shaft 3 is mounted to a fixed part such as a case, not shown, orthe like so as neither to rotate nor axially move. The nut 2 is disposedso as to rotate and axially move with respect to the screw shaft 3.

The gear ring 7 comprises an annular body 8 made of metal and having asectional upper half portion substantially in the shape of a reversehorseshoe and a gear 9 made of resin and integrally formed in the outerperipheral surface of an external cylindrical portion 81 in the annularbody 8. The nut 2 is inserted with respect to the inner peripheralsurface of the external cylindrical portion 81 in the annular body 8 ofthe gear ring 7. As shown in FIG. 5, a serration 82 provided in theinner peripheral surface of the external cylindrical portion 81 on thebase side thereof and a serration 23 provided in the outer peripheralsurface of the nut 2 on the back side in the insertion direction thereofare engaged with each other to thereby connect the gear ring 7 and thenut 2 in a relatively non-rotatable manner. The foregoing rollingbearing, not shown, is fitted to the inner peripheral surface of aninternal cylindrical portion 83 in the annular body 8 of the gear ring7.

The retainer ring 5 is mounted on the screw shaft 3 so as to besubstantially axially immovable and relatively rotatable. For thatreason, a reduced diameter portion 35 and a radially inward flange 52are provided respectively on the free end side of the screw shaft 3 andon one end of the retainer ring 5. The flange 52 of the retainer ring 5is fitted to the reduced diameter portion 35 of the screw shaft 3. Asnap ring 10 is engaged with a peripheral groove provided on the reduceddiameter portion 35 of the screw shaft 3. However, the snap ring 10 ismounted at a position distant from a step wall surface 36 formed on theborder of the reduced diameter portion 35 of the screw shaft 3 and theportion where the thread groove 21 is formed. The flange 52 of theretainer ring 5 is disposed, with a slight axial play, between the snapring 10 and the step wall surface 36. Accordingly, the retainer ring 5is substantially axially immovable with respect to the screw shaft 3 butrelatively rotatable.

The assembling steps of the ball screw device 1 are hereinafterdescribed.

First, the retainer ring 5 is mounted on the screw shaft 3, and then theball pockets 51 of the retainer ring 5 are coated with grease to theextent of being thereby infilled. A required number of the balls 4 areplaced in the ball pockets 51. The grease used here has enough viscosityto prevent the balls 4 from falling by their own weight and serves toretain the balls 4 inside the ball pockets 51. When the foregoing stepshave been done, the retainer ring 5, which been arranged not to rotatewith respect to the screw shaft 3, is incorporated in the nut 2.

The operation of the foregoing ball screw device 1 is hereinafterdescribed.

When the gear ring 7 and the nut 2 are rotated by driving a motor, notshown, in one direction, the nut 2 itself is, being rotated, guided bythe screw shaft 3 to be thereby linearly moved in one axial direction sothat, for example, the state shown in FIG. 1 is shifted to the stateshown in FIG. 2. When the motor is driven in the opposite direction, thenut 2 is, being rotated in the opposite direction as well, moved in theopposite axial direction so that, for example, the state shown in FIG. 2is shifted to the state shown in FIG. 1.

Thus, when the nut 2 is axially reciprocated, an axially overlappingregion of the nut 2 and the screw shaft 3 increases or decreases. Theballs 4 are guided by the retainer ring 5 so as to roll and therebycirculate respectively in the thread grooves 31 a and 31 b of the screwshaft 3, which are formed into independent closed loops by the ballcirculation grooves 33 and 34, so that spiral movement of the nut 2 issmoothly guided, while the balls 4 are unfailingly prevented fromslipping out when the nut 2 reciprocates in a predetermined stroke.

As described, the embodiment of the present invention has such astructure as follows: the thread grooves 31 a and 31 b axially adjacentto one another in the screw shaft 3 are formed into independent closedloops; the balls 4 are rolled and thereby circulated in the closedloops; and two ball circulation grooves 33 and 34, instead of theconventional deflector, are provided at the ridge 32 of the screw shaft3. Therefore, in this embodiment, the number of parts to be used can bereduced compared to the conventional products, which advantageouslysaves such labor as forming a through hole for fitting the deflector tothe nut 2 and mounting the deflector thereto. As a result of that, inthis embodiment, manufacturing cost can be reduced compared to the useof the deflector. Further, in this embodiment, it is unnecessary,contrary to the conventional products, to adjust the positioning of thedeflector with respect to the ball circulation grooves and the threadgrooves, clearing the possibility of any inferior quality caused by sucha deviated positioning in case it occurs by any chance.

As described earlier, when two ball circulation grooves 33 and 34 areprovided in a substantially identical phase and axially adjacent to oneanother, as shown in FIG. 6, so that the thread grooves 31 a and 31 b ofthe screw shaft 3 can be axially closely spaced so as to advantageouslyreduce an area which is thereby axially occupied. However, in this case,the balls 4 disposed in the ball circulation grooves 33 and 34 can besubject to neither a radial load nor an axial load, and thereforeproviding two ball circulation grooves 33 and 34 circumferentially andaxially in the vicinity will create a non-load bearing area in apredetermined angle on the circumference of the thread grooves 31 a and31 b. However, by shortening the axial dimensions of the nut 2 and thescrew shaft 3 and setting the outer diameters thereof at a larger valuein accordance with the foregoing embodiment, a region in an angle θ,which is covered by the ball circulation grooves 33 and 34 on thecircumference of the thread grooves 31 a and 31 b, can be smaller, asshown in FIG. 7, and a smaller number of the balls 4 is required to bedisposed in the ball circulation grooves 33 and 34. Accordingly, thedecrease of load bearing performance can be controlled so as toeliminate possible trouble in practical use.

The present invention is not limited to the described embodiment of theinvention, and different applications and modifications are possibleoptions.

For example, the ball screw device 1 may be used in such manner that oneof the nut 2 and the screw shaft 3 is rotated to axially move the other,that is a first mode of use, or in such manner that one of the nut 2 andthe screw shaft 3 is axially moved to rotate the other, that is a secondmode of use. The first mode of use is referred to as a forwardefficiency for converting a torque into a thrust and four examplesthereof follow. The second mode of use is referred as a reverseefficiency for converting a thrust into a torque and four examplesthereof follow.

1. A First Mode of Use

{circle around (1)} As described heretofore, the nut 2 is, beingrotated, axially moved. In this case, the screw shaft 3 is arranged tobe non-rotatable and axially immovable to have the nut 2 rotate.

{circle around (2)} The nut 2 is, without being rotated, axially moved.In this case, the screw shaft 3 is arranged to be axially immovable,while the nut 2 is arranged to be non-rotatable to have the screw shaft3 rotate.

{circle around (3)} The screw shaft 3 can be, being rotated, axiallymoved. In this case, the nut 2 is arranged to be non-rotatable andaxially immovable to have the screw shaft 3 rotate.

{circle around (4)} The screw shaft 3 is, without being rotated, axiallymoved. In this case, the screw shaft 3 is arranged to be non-rotatable,while the nut 2 is arranged to be axially immovable and rotate.

2. A Second Mode of Use

{circle around (1)} The nut 2 is axially immovably rotated. In thiscase, the nut 2 is arranged to be axially immovable, while the screwshaft 3 is arranged to be non-rotatable and axially moved.

{circle around (2)} The nut 2 is, being axially moved, rotated. In thiscase, the screw shaft 3 is arranged to be axially immovable andnon-rotatable to have the nut 2 axially move.

{circle around (3)} The screw shaft 3 is axially immovably rotated. Inthis case, the screw shaft 3 is arranged to be axially immovable, whilethe nut 2 is arranged to be non-rotatable and axially moved.

{circle around (4)} The screw shaft 3 is, being axially moved, rotated.In this case, the nut 2 is arranged to be axially immovable andnon-rotatable to have the screw shaft 3 axially move.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a method of converting arotational movement into a linear movement and vise versa in machinetools, semiconductor devices, precision position tables, robots andconveying equipment.

1. A ball screw device comprising: a nut having a nut thread groove inan inner peripheral surface thereof defining a ridge in the innerperipheral surface; a screw shaft having an outer peripheral surface anda plurality of screw shaft thread grooves each formed in a continuousfirst helical direction in the outer peripheral surface and axiallyadjacent to one another, each of said screw shaft thread grooves havingan upstream end and a downstream end; a plurality of balls interposedbetween the screw shaft thread grooves and the nut thread groove; saidscrew shall having ball circulation grooves each adjoining and couplingthe downstream end of one of the screw shaft thread grooves and theupstream end of the one of the screw shaft thread grooves so that theballs are returned to the upstream end from the downstream end so as tobe thereby circulated, the ball circulation grooves being formed in theouter peripheral surface of the screw shaft and substantially in acommon angular segment of the screw shaft defined by the upstream endsand the downstream ends of the screw shaft thread grooves, and the ballcirculation grooves being axially adjacent to one another; and aretainer ring for rotatably retaining each of the balls incircumferentially spaced positions around a circumference of said screwshaft, said retainer ring being mounted to an outer periphery of thescrew shaft so as to rotate relative said screw shaft and remain in anaxially fixed position relative said screw shaft.
 2. The ball screwdevice as claimed in claim 1, wherein the plurality of screw shaftthread grooves are each of substantially one turn.
 3. The ball screwdevice as claimed in claim 1, wherein the ball circulation groovesradially inwardly sink the balls rolled to the downstream ends of thescrew shaft thread grooves of the screw shaft and direct the balls totravel over ridge of the nut and reach the upstream ends of the screwshaft thread grooves.
 4. The ball screw device of claim 1 wherein theretainer ring retains each of the balls such that the halls inrespective ones of the screw shaft grooves are angularly positionedrelative the respective ones of the screw shaft grooves and the ballcirculation grooves at positions common to corresponding other ones ofthe balls in other ones of the screw shaft grooves such that thecorresponding balls move in like phase positions through respective onesof the screw shaft grooves to undergo like simultaneous motion in thescrew shaft grooves and the ball circulation grooves.
 5. The ball screwdevice of claim 4 wherein the retainer defines axially extending slotswithin which the balls are retained circumferentially separated fromeach other.
 6. The ball screw device of claim 5 wherein the axiallyextending slots individually retain a first ball situated a first shaftthread groove of said plurality of screw shaft thread grooves and asecond ball situated in a second screw shaft thread groove of saidplurality of screw shaft thread grooves, said first and second ballsbeing of said balls interposed between the screw shaft thread groovesarid the nut thread groove and being said corresponding balls that movein like phase positions.
 7. The ball screw device of claim 6 wherein theouter periphery of the screw shaft, to which the retainer is mounted,includes a circumferential step face and a circumferential groove spacedfrom said circumferential step face and a snap ring disposed in thecircumferential groove, and said retainer includes a radially inwardlyextending portion disposed between said circumferential step face andsaid snap ring so as to fixedly axially position said retainer relativeto said screw shaft and allow the retainer to rotate relative said screwshaft.
 8. The ball screw device as claimed in claim 7, wherein theplurality of screw shaft thread grooves are each of substantially nearlyone turn.
 9. The ball screw device as claimed in claim 8, wherein theball circulation grooves radially inwardly sink the balls rolled to thedownstream ends of the screw shaft thread grooves of the screw shaft anddirect the balls to travel over the ridge of the nut and reach theupstream ends of the screw shaft thread grooves.
 10. The ball screwdevice of claim 1 wherein the retainer defines slots within which theballs are retained circumferentially separated from each other.
 11. Theball screw device of claim 10 wherein the slots individually retain afirst ball situated a first shaft thread groove of said plurality ofscrew shaft thread grooves and a second ball situated in a second screwshaft thread groove of said plurality of screw shaft thread grooves,said first and second balls being of said balls interposed between thescrew shaft thread grooves and the nut thread groove.
 12. The ball screwdevice of claim 11 wherein the outer periphery of the screw shaft, towhich the retainer is mounted, includes a circumferential step face anda circumferential groove spaced from said circumferential step face anda snap ring disposed in the circumferential groove, and said retainerincludes a radially inwardly extending portion disposed between saidcircumferential step face and said snap ring so as to fixedly axiallyposition said retainer relative to said screw shaft and allow theretainer to route relative said screw shaft.
 13. The ball screw deviceas claimed in claim 12, wherein the plurality of screw shaft threadgrooves are each of substantially nearly one turn.
 14. The ball screwdevice as claimed in claim 13, wherein the ball circulation groovesradially inwardly sink the balls rolled to the downstream ends of thescrew shaft thread grooves of the screw shaft and direct the balls totravel over the ridge of the nut and reach the upstream ends of thescrew shaft thread grooves.
 15. The ball screw device of claim 1 whereinthe outer periphery of the screw shaft, to which the retainer ismounted, includes a circumferential step face and a circumferentialgroove spaced from said circumferential step face and a snap ringdisposed in the circumferential groove, and said retainer includes aradially inwardly extending portion disposed between saidcircumferential step face and said snap ring so as to fixedly axiallyposition said retainer relative to said screw shaft and allow theretainer to rotate relative said screw shaft.
 16. The ball screw deviceas claimed in claim 15, wherein the plurality of screw shaft threadgrooves arc each of substantially nearly one turn.
 17. The ball screwdevice as claimed in claim 16, wherein the ball circulation groovesradially inwardly sink the balls rolled to the downstream ends of thescrew shalt thread grooves of the screw shaft and direct the balls totravel over the ridge of the nut and reach the upstream ends of thescrew shaft thread grooves.